-LRB- CNN -RRB- -- Hours after losing Victoria Azarenka to injury , organizers of the Dubai Tennis Championships were dealt another major blow on Wednesday when Serena Williams also pulled out of the event .

The 31-year-old Williams , who became the oldest women 's tennis player to be crowned world No. 1 on Monday , said she was forced out because of a back injury .

`` I 've just had some back problems the past couple of weeks , '' the American told an impromptu press conference .

`` I thought it would get better as the week went on but it did n't . I do n't want to keep pushing it and make it worse . ''

Williams took to the court to apologize to fans who had arrived for her second-round match with Marion Bartoli of France .

The absences of both Williams and previous No. 1 Azarenka represent a severe dent to a popular $ 2 million tournament which had already been without the sport 's two reported highest earners , Maria Sharapova and Li Na -LRB- according to Forbes magazine -RRB- .

It is also a blow to the Women 's Tennis Association , which has worked hard to introduce incentives and regulations to reduce the number of withdrawals from its events .

Wednesday 's unexpected withdrawal follows that of second-ranked Azarenka , which ensured that Williams will hang on to top spot until the Sony Ericsson Open , which starts in Miami on March 18 .

Williams said her back also troubled her in last week 's Qatar Open , where she was beaten in the final by the Belorussian , who withdrew from the Dubai championships with a foot injury .

The 15-time grand slam winner added that , having returned to the pinnacle after two and a half years during which her life and career were threatened following a freak foot injury in 2010 , being No. 1 is no longer her primary goal .

`` OK , I have done it , let 's focus on my next goals which are the grand slams , '' Williams said , admitting that she already had at least half an eye on the French Open in Paris , starting on May 26 .

`` I really want to continue doing really well in those . ''

After winning Wimbledon and the US Open last year , Williams ' total of grand slam titles is only three fewer than Martina Navratilova and Chris Evert , who together are second on the all-time list behind Steffi Graf with 22 .

Williams ' absence increases the chances of world No. 4 Agnieszka Radwanska making a successful defense of her title , which began with the Pole grinding out a 7 - , 6-3 win over Yulia Putintseva , a promising 18-year-old wild card entry from Kazakhstan .

Bartoli , who received a wild-card invite after making a late entry , will face former world No. 1 Caroline Wozniacki in the quarterfinals .

The Dane , now ranked 10th and a winner of the tournament in 2011 , progressed with a 6-0 6-1 drubbing of China 's former Wimbledon semifinalist Zheng Jie .

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Serena Williams pulls out minutes before scheduled clash with Marion Bartoli

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World No. 1 says back problem has troubled her for the past fortnight

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American targets next month 's Sony Ericsson Open in Miami for comeback